1. Field of the Invention
The present invention relates to an optical element including a lens base plate in which a lens of a high refractive index is embedded into a base plate and an optical pickup using this optical element.
2. Description of the Related Art
In recent years, micro-lenses are widely used in various fields, such as optical communication and an optical head of an optical pickup for use in recording and reproducing an information signal on and from an optical recording medium, e.g., optical disk. It is strongly requested that these micro-lenses should have a large NA (numerical aperture) and should be formed as aspherical micro-lenses.
In micro-lenses for use in optical communication, an optical pickup and the like, a sheet forming method using a mold has hitherto been used as a manufacturing method for mass-producing micro-lenses inexpensively.
When micro-lenses are used in the field of optical pickup and so on, there is an increasing demand of reducing a diameter of lens and increasing a radius of curvature of a lens to increase a numerical aperture of a lens.
However, in order to manufacture a lens with a small diameter and of which the radius of curvature is large, it becomes difficult to produce a mold for use in sheet forming.
For this reason, as a micro-lens manufacturing method, there have been proposed a photolithography technique and manufacturing methods proposed by the same assignee of the present application (see Japanese patent applications Nos. 2000-132897, 2000-189729, 2000-189730, 2000-305122, 2001-37366, 2001-51736).
However, according to these previously-proposed manufacturing methods, it is very difficult to make both surfaces of a lens become aspherical surfaces and it is also very difficult to increase a thickness of a lens from a technique standpoint.
As a result, lenses manufactured by these manufacturing methods are shaped like thin flat convex aspherical lenses, i.e. one major surface is shaped like an aspherical shape and another major surface is shaped like a flat surface.
In this lens shaped like the flat convex aspherical lens, if such lens is designed such that spherical aberration produced on the optical axis on the major surface side that is the aspherical surface may be corrected, there then arises a problem that an amount of comatic aberration produced when the optical axis of incident light is inclined relative to the lens will not be decreased at the same time. The reason for this is that, while the comatic aberration produced when the optical axis of the incident light is inclined can be corrected on the major surface that is the aspherical surface, the comatic aberration cannot be corrected on the major surface that is the flat surface and that the aspherical surface is optimized in order to correct the spherical aberration so that the comatic aberration cannot be corrected completely.